Note: The code samples in this tutorial use doctest to make sure that
they actually work. Since some code samples behave differently under Linux,
Windows, or Mac OS X, they contain doctest directives in comments.

Note: Some code samples reference the ctypes c_int type. This type is
an alias for the c_long type on 32-bit systems. So, you should not be
confused if c_long is printed if you would expect c_int —
they are actually the same type.

ctypes exports the cdll, and on Windows windll and oledll
objects, for loading dynamic link libraries.

You load libraries by accessing them as attributes of these objects. cdll
loads libraries which export functions using the standard cdecl calling
convention, while windll libraries call functions using the stdcall
calling convention. oledll also uses the stdcall calling convention, and
assumes the functions return a Windows HRESULT error code. The error
code is used to automatically raise an OSError exception when the
function call fails.

Changed in version 3.3: Windows errors used to raise WindowsError, which is now an alias
of OSError.

Here are some examples for Windows. Note that msvcrt is the MS standard C
library containing most standard C functions, and uses the cdecl calling
convention:

On Linux, it is required to specify the filename including the extension to
load a library, so attribute access can not be used to load libraries. Either the
LoadLibrary() method of the dll loaders should be used, or you should load
the library by creating an instance of CDLL by calling the constructor:

Note that win32 system dlls like kernel32 and user32 often export ANSI
as well as UNICODE versions of a function. The UNICODE version is exported with
an W appended to the name, while the ANSI version is exported with an A
appended to the name. The win32 GetModuleHandle function, which returns a
module handle for a given module name, has the following C prototype, and a
macro is used to expose one of them as GetModuleHandle depending on whether
UNICODE is defined or not:

windll does not try to select one of them by magic, you must access the
version you need by specifying GetModuleHandleA or GetModuleHandleW
explicitly, and then call it with bytes or string objects respectively.

Sometimes, dlls export functions with names which aren’t valid Python
identifiers, like "??2@YAPAXI@Z". In this case you have to use
getattr() to retrieve the function:

>>> getattr(cdll.msvcrt,"??2@YAPAXI@Z")<_FuncPtr object at 0x...>>>>

On Windows, some dlls export functions not by name but by ordinal. These
functions can be accessed by indexing the dll object with the ordinal number:

You can call these functions like any other Python callable. This example uses
the time() function, which returns system time in seconds since the Unix
epoch, and the GetModuleHandleA() function, which returns a win32 module
handle.

This example calls both functions with a NULL pointer (None should be used
as the NULL pointer):

ctypes tries to protect you from calling functions with the wrong number
of arguments or the wrong calling convention. Unfortunately this only works on
Windows. It does this by examining the stack after the function returns, so
although an error is raised the function has been called:

There are, however, enough ways to crash Python with ctypes, so you
should be careful anyway. The faulthandler module can be helpful in
debugging crashes (e.g. from segmentation faults produced by erroneous C library
calls).

None, integers, bytes objects and (unicode) strings are the only native
Python objects that can directly be used as parameters in these function calls.
None is passed as a C NULL pointer, bytes objects and strings are passed
as pointer to the memory block that contains their data (char* or
wchar_t*). Python integers are passed as the platforms default C
int type, their value is masked to fit into the C type.

Before we move on calling functions with other parameter types, we have to learn
more about ctypes data types.

Assigning a new value to instances of the pointer types c_char_p,
c_wchar_p, and c_void_p changes the memory location they
point to, not the contents of the memory block (of course not, because Python
bytes objects are immutable):

You should be careful, however, not to pass them to functions expecting pointers
to mutable memory. If you need mutable memory blocks, ctypes has a
create_string_buffer() function which creates these in various ways. The
current memory block contents can be accessed (or changed) with the raw
property; if you want to access it as NUL terminated string, use the value
property:

The create_string_buffer() function replaces the c_buffer() function
(which is still available as an alias), as well as the c_string() function
from earlier ctypes releases. To create a mutable memory block containing
unicode characters of the C type wchar_t use the
create_unicode_buffer() function.

As has been mentioned before, all Python types except integers, strings, and
bytes objects have to be wrapped in their corresponding ctypes type, so
that they can be converted to the required C data type:

You can also customize ctypes argument conversion to allow instances of
your own classes be used as function arguments. ctypes looks for an
_as_parameter_ attribute and uses this as the function argument. Of
course, it must be one of integer, string, or bytes:

It is possible to specify the required argument types of functions exported from
DLLs by setting the argtypes attribute.

argtypes must be a sequence of C data types (the printf function is
probably not a good example here, because it takes a variable number and
different types of parameters depending on the format string, on the other hand
this is quite handy to experiment with this feature):

If you have defined your own classes which you pass to function calls, you have
to implement a from_param() class method for them to be able to use them
in the argtypes sequence. The from_param() class method receives
the Python object passed to the function call, it should do a typecheck or
whatever is needed to make sure this object is acceptable, and then return the
object itself, its _as_parameter_ attribute, or whatever you want to
pass as the C function argument in this case. Again, the result should be an
integer, string, bytes, a ctypes instance, or an object with an
_as_parameter_ attribute.

You can also use a callable Python object (a function or a class for example) as
the restype attribute, if the foreign function returns an integer. The
callable will be called with the integer the C function returns, and the
result of this call will be used as the result of your function call. This is
useful to check for error return values and automatically raise an exception:

WinError is a function which will call Windows FormatMessage() api to
get the string representation of an error code, and returns an exception.
WinError takes an optional error code parameter, if no one is used, it calls
GetLastError() to retrieve it.

Please note that a much more powerful error checking mechanism is available
through the errcheck attribute; see the reference manual for details.

Sometimes a C api function expects a pointer to a data type as parameter,
probably to write into the corresponding location, or if the data is too large
to be passed by value. This is also known as passing parameters by reference.

ctypes exports the byref() function which is used to pass parameters
by reference. The same effect can be achieved with the pointer() function,
although pointer() does a lot more work since it constructs a real pointer
object, so it is faster to use byref() if you don’t need the pointer
object in Python itself:

Structures and unions must derive from the Structure and Union
base classes which are defined in the ctypes module. Each subclass must
define a _fields_ attribute. _fields_ must be a list of
2-tuples, containing a field name and a field type.

The field type must be a ctypes type like c_int, or any other
derived ctypes type: structure, union, array, pointer.

Here is a simple example of a POINT structure, which contains two integers named
x and y, and also shows how to initialize a structure in the constructor:

ctypes does not support passing unions or structures with bit-fields
to functions by value. While this may work on 32-bit x86, it’s not
guaranteed by the library to work in the general case. Unions and
structures with bit-fields should always be passed to functions by pointer.

By default, Structure and Union fields are aligned in the same way the C
compiler does it. It is possible to override this behavior be specifying a
_pack_ class attribute in the subclass definition. This must be set to a
positive integer and specifies the maximum alignment for the fields. This is
what #pragmapack(n) also does in MSVC.

ctypes uses the native byte order for Structures and Unions. To build
structures with non-native byte order, you can use one of the
BigEndianStructure, LittleEndianStructure,
BigEndianUnion, and LittleEndianUnion base classes. These
classes cannot contain pointer fields.

Pointer instances are created by calling the pointer() function on a
ctypes type:

>>> fromctypesimport*>>> i=c_int(42)>>> pi=pointer(i)>>>

Pointer instances have a contents attribute which returns the object to
which the pointer points, the i object above:

>>> pi.contentsc_long(42)>>>

Note that ctypes does not have OOR (original object return), it constructs a
new, equivalent object each time you retrieve an attribute:

>>> pi.contentsisiFalse>>> pi.contentsispi.contentsFalse>>>

Assigning another c_int instance to the pointer’s contents attribute
would cause the pointer to point to the memory location where this is stored:

>>> i=c_int(99)>>> pi.contents=i>>> pi.contentsc_long(99)>>>

Pointer instances can also be indexed with integers:

>>> pi[0]99>>>

Assigning to an integer index changes the pointed to value:

>>> print(i)c_long(99)>>> pi[0]=22>>> print(i)c_long(22)>>>

It is also possible to use indexes different from 0, but you must know what
you’re doing, just as in C: You can access or change arbitrary memory locations.
Generally you only use this feature if you receive a pointer from a C function,
and you know that the pointer actually points to an array instead of a single
item.

Behind the scenes, the pointer() function does more than simply create
pointer instances, it has to create pointer types first. This is done with the
POINTER() function, which accepts any ctypes type, and returns a
new type:

Usually, ctypes does strict type checking. This means, if you have
POINTER(c_int) in the argtypes list of a function or as the type of
a member field in a structure definition, only instances of exactly the same
type are accepted. There are some exceptions to this rule, where ctypes accepts
other objects. For example, you can pass compatible array instances instead of
pointer types. So, for POINTER(c_int), ctypes accepts an array of c_int:

In addition, if a function argument is explicitly declared to be a pointer type
(such as POINTER(c_int)) in argtypes, an object of the pointed
type (c_int in this case) can be passed to the function. ctypes will apply
the required byref() conversion in this case automatically.

To set a POINTER type field to NULL, you can assign None:

>>> bar.values=None>>>

Sometimes you have instances of incompatible types. In C, you can cast one type
into another type. ctypes provides a cast() function which can be
used in the same way. The Bar structure defined above accepts
POINTER(c_int) pointers or c_int arrays for its values field,
but not instances of other types:

The cast() function can be used to cast a ctypes instance into a pointer
to a different ctypes data type. cast() takes two parameters, a ctypes
object that is or can be converted to a pointer of some kind, and a ctypes
pointer type. It returns an instance of the second argument, which references
the same memory block as the first argument:

qsort() must be called with a pointer to the data to sort, the number of
items in the data array, the size of one item, and a pointer to the comparison
function, the callback. The callback will then be called with two pointers to
items, and it must return a negative integer if the first item is smaller than
the second, a zero if they are equal, and a positive integer otherwise.

So our callback function receives pointers to integers, and must return an
integer. First we create the type for the callback function:

>>> CMPFUNC=CFUNCTYPE(c_int,POINTER(c_int),POINTER(c_int))>>>

To get started, here is a simple callback that shows the values it gets
passed:

Make sure you keep references to CFUNCTYPE() objects as long as they
are used from C code. ctypes doesn’t, and if you don’t, they may be
garbage collected, crashing your program when a callback is made.

Also, note that if the callback function is called in a thread created
outside of Python’s control (e.g. by the foreign code that calls the
callback), ctypes creates a new dummy Python thread on every invocation. This
behavior is correct for most purposes, but it means that values stored with
threading.local will not survive across different callbacks, even when
those calls are made from the same C thread.

Some shared libraries not only export functions, they also export variables. An
example in the Python library itself is the Py_OptimizeFlag, an integer
set to 0, 1, or 2, depending on the -O or -OO flag given on
startup.

ctypes can access values like this with the in_dll() class methods of
the type. pythonapi is a predefined symbol giving access to the Python C
api:

If the interpreter would have been started with -O, the sample would
have printed c_long(1), or c_long(2) if -OO would have been
specified.

An extended example which also demonstrates the use of pointers accesses the
PyImport_FrozenModules pointer exported by Python.

Quoting the docs for that value:

This pointer is initialized to point to an array of struct_frozen
records, terminated by one whose members are all NULL or zero. When a frozen
module is imported, it is searched in this table. Third-party code could play
tricks with this to provide a dynamically created collection of frozen modules.

So manipulating this pointer could even prove useful. To restrict the example
size, we show only how this table can be read with ctypes:

Since table is a pointer to the array of struct_frozen records, we
can iterate over it, but we just have to make sure that our loop terminates,
because pointers have no size. Sooner or later it would probably crash with an
access violation or whatever, so it’s better to break out of the loop when we
hit the NULL entry:

The fact that standard Python has a frozen module and a frozen package
(indicated by the negative size member) is not well known, it is only used for
testing. Try it out with import__hello__ for example.

Hm. We certainly expected the last statement to print 3412. What
happened? Here are the steps of the rc.a,rc.b=rc.b,rc.a line above:

>>> temp0,temp1=rc.b,rc.a>>> rc.a=temp0>>> rc.b=temp1>>>

Note that temp0 and temp1 are objects still using the internal buffer of
the rc object above. So executing rc.a=temp0 copies the buffer
contents of temp0 into rc ‘s buffer. This, in turn, changes the
contents of temp1. So, the last assignment rc.b=temp1, doesn’t have
the expected effect.

Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays
doesn’t copy the sub-object, instead it retrieves a wrapper object accessing
the root-object’s underlying buffer.

Another example that may behave different from what one would expect is this:

Why is it printing False? ctypes instances are objects containing a memory
block plus some descriptors accessing the contents of the memory.
Storing a Python object in the memory block does not store the object itself,
instead the contents of the object is stored. Accessing the contents again
constructs a new Python object each time!

ctypes provides some support for variable-sized arrays and structures.

The resize() function can be used to resize the memory buffer of an
existing ctypes object. The function takes the object as first argument, and
the requested size in bytes as the second argument. The memory block cannot be
made smaller than the natural memory block specified by the objects type, a
ValueError is raised if this is tried:

When programming in a compiled language, shared libraries are accessed when
compiling/linking a program, and when the program is run.

The purpose of the find_library() function is to locate a library in a way
similar to what the compiler does (on platforms with several versions of a
shared library the most recent should be loaded), while the ctypes library
loaders act like when a program is run, and call the runtime loader directly.

The ctypes.util module provides a function which can help to determine
the library to load.

ctypes.util.find_library(name)

Try to find a library and return a pathname. name is the library name without
any prefix like lib, suffix like .so, .dylib or version number (this
is the form used for the posix linker option -l). If no library can
be found, returns None.

The exact functionality is system dependent.

On Linux, find_library() tries to run external programs
(/sbin/ldconfig, gcc, and objdump) to find the library file. It
returns the filename of the library file. Here are some examples:

On Windows, find_library() searches along the system search path, and
returns the full pathname, but since there is no predefined naming scheme a call
like find_library("c") will fail and return None.

If wrapping a shared library with ctypes, it may be better to determine
the shared library name at development time, and hardcode that into the wrapper
module instead of using find_library() to locate the library at runtime.

Windows only: Instances of this class represent loaded shared libraries,
functions in these libraries use the stdcall calling convention, and are
assumed to return the windows specific HRESULT code. HRESULT
values contain information specifying whether the function call failed or
succeeded, together with additional error code. If the return value signals a
failure, an OSError is automatically raised.

Instances of this class behave like CDLL instances, except that the
Python GIL is not released during the function call, and after the function
execution the Python error flag is checked. If the error flag is set, a Python
exception is raised.

Thus, this is only useful to call Python C api functions directly.

All these classes can be instantiated by calling them with at least one
argument, the pathname of the shared library. If you have an existing handle to
an already loaded shared library, it can be passed as the handle named
parameter, otherwise the underlying platforms dlopen or LoadLibrary
function is used to load the library into the process, and to get a handle to
it.

The mode parameter can be used to specify how the library is loaded. For
details, consult the dlopen(3) manpage, on Windows, mode is
ignored.

The use_errno parameter, when set to True, enables a ctypes mechanism that
allows to access the system errno error number in a safe way.
ctypes maintains a thread-local copy of the systems errno
variable; if you call foreign functions created with use_errno=True then the
errno value before the function call is swapped with the ctypes private
copy, the same happens immediately after the function call.

The function ctypes.get_errno() returns the value of the ctypes private
copy, and the function ctypes.set_errno() changes the ctypes private copy
to a new value and returns the former value.

The use_last_error parameter, when set to True, enables the same mechanism for
the Windows error code which is managed by the GetLastError() and
SetLastError() Windows API functions; ctypes.get_last_error() and
ctypes.set_last_error() are used to request and change the ctypes private
copy of the windows error code.

ctypes.RTLD_GLOBAL

Flag to use as mode parameter. On platforms where this flag is not available,
it is defined as the integer zero.

ctypes.RTLD_LOCAL

Flag to use as mode parameter. On platforms where this is not available, it
is the same as RTLD_GLOBAL.

ctypes.DEFAULT_MODE

The default mode which is used to load shared libraries. On OSX 10.3, this is
RTLD_GLOBAL, otherwise it is the same as RTLD_LOCAL.

Instances of these classes have no public methods. Functions exported by the
shared library can be accessed as attributes or by index. Please note that
accessing the function through an attribute caches the result and therefore
accessing it repeatedly returns the same object each time. On the other hand,
accessing it through an index returns a new object each time:

>>> libc.time==libc.timeTrue>>> libc['time']==libc['time']False

The following public attributes are available, their name starts with an
underscore to not clash with exported function names:

Shared libraries can also be loaded by using one of the prefabricated objects,
which are instances of the LibraryLoader class, either by calling the
LoadLibrary() method, or by retrieving the library as attribute of the
loader instance.

__getattr__() has special behavior: It allows to load a shared library by
accessing it as attribute of a library loader instance. The result is cached,
so repeated attribute accesses return the same library each time.

An instance of PyDLL that exposes Python C API functions as
attributes. Note that all these functions are assumed to return C
int, which is of course not always the truth, so you have to assign
the correct restype attribute to use these functions.

As explained in the previous section, foreign functions can be accessed as
attributes of loaded shared libraries. The function objects created in this way
by default accept any number of arguments, accept any ctypes data instances as
arguments, and return the default result type specified by the library loader.
They are instances of a private class:

Assign a ctypes type to specify the result type of the foreign function.
Use None for void, a function not returning anything.

It is possible to assign a callable Python object that is not a ctypes
type, in this case the function is assumed to return a C int, and
the callable will be called with this integer, allowing to do further
processing or error checking. Using this is deprecated, for more flexible
post processing or error checking use a ctypes data type as
restype and assign a callable to the errcheck attribute.

Assign a tuple of ctypes types to specify the argument types that the
function accepts. Functions using the stdcall calling convention can
only be called with the same number of arguments as the length of this
tuple; functions using the C calling convention accept additional,
unspecified arguments as well.

When a foreign function is called, each actual argument is passed to the
from_param() class method of the items in the argtypes
tuple, this method allows to adapt the actual argument to an object that
the foreign function accepts. For example, a c_char_p item in
the argtypes tuple will convert a string passed as argument into
a bytes object using ctypes conversion rules.

New: It is now possible to put items in argtypes which are not ctypes
types, but each item must have a from_param() method which returns a
value usable as argument (integer, string, ctypes instance). This allows
to define adapters that can adapt custom objects as function parameters.

Foreign functions can also be created by instantiating function prototypes.
Function prototypes are similar to function prototypes in C; they describe a
function (return type, argument types, calling convention) without defining an
implementation. The factory functions must be called with the desired result
type and the argument types of the function.

The returned function prototype creates functions that use the standard C
calling convention. The function will release the GIL during the call. If
use_errno is set to True, the ctypes private copy of the system
errno variable is exchanged with the real errno value before
and after the call; use_last_error does the same for the Windows error
code.

Windows only: The returned function prototype creates functions that use the
stdcall calling convention, except on Windows CE where
WINFUNCTYPE() is the same as CFUNCTYPE(). The function will
release the GIL during the call. use_errno and use_last_error have the
same meaning as above.

Returns a foreign function exported by a shared library. func_spec must
be a 2-tuple (name_or_ordinal,library). The first item is the name of
the exported function as string, or the ordinal of the exported function
as small integer. The second item is the shared library instance.

prototype(vtbl_index, name[, paramflags[, iid]])

Returns a foreign function that will call a COM method. vtbl_index is
the index into the virtual function table, a small non-negative
integer. name is name of the COM method. iid is an optional pointer to
the interface identifier which is used in extended error reporting.

COM methods use a special calling convention: They require a pointer to
the COM interface as first argument, in addition to those parameters that
are specified in the argtypes tuple.

The optional paramflags parameter creates foreign function wrappers with much
more functionality than the features described above.

paramflags must be a tuple of the same length as argtypes.

Each item in this tuple contains further information about a parameter, it must
be a tuple containing one, two, or three items.

The first item is an integer containing a combination of direction
flags for the parameter:

1

Specifies an input parameter to the function.

2

Output parameter. The foreign function fills in a value.

4

Input parameter which defaults to the integer zero.

The optional second item is the parameter name as string. If this is specified,
the foreign function can be called with named parameters.

The optional third item is the default value for this parameter.

This example demonstrates how to wrap the Windows MessageBoxA function so
that it supports default parameters and named arguments. The C declaration from
the windows header file is this:

A second example demonstrates output parameters. The win32 GetWindowRect
function retrieves the dimensions of a specified window by copying them into
RECT structure that the caller has to supply. Here is the C declaration:

Functions with output parameters will automatically return the output parameter
value if there is a single one, or a tuple containing the output parameter
values when there are more than one, so the GetWindowRect function now returns a
RECT instance, when called.

Output parameters can be combined with the errcheck protocol to do
further output processing and error checking. The win32 GetWindowRect api
function returns a BOOL to signal success or failure, so this function could
do the error checking, and raises an exception when the api call failed:

If the errcheck function returns the argument tuple it receives
unchanged, ctypes continues the normal processing it does on the output
parameters. If you want to return a tuple of window coordinates instead of a
RECT instance, you can retrieve the fields in the function and return them
instead, the normal processing will no longer take place:

This function is similar to the cast operator in C. It returns a new instance
of type which points to the same memory block as obj. type must be a
pointer type, and obj must be an object that can be interpreted as a
pointer.

This function creates a mutable character buffer. The returned object is a
ctypes array of c_char.

init_or_size must be an integer which specifies the size of the array, or a
bytes object which will be used to initialize the array items.

If a bytes object is specified as first argument, the buffer is made one item
larger than its length so that the last element in the array is a NUL
termination character. An integer can be passed as second argument which allows
to specify the size of the array if the length of the bytes should not be used.

This function creates a mutable unicode character buffer. The returned object is
a ctypes array of c_wchar.

init_or_size must be an integer which specifies the size of the array, or a
string which will be used to initialize the array items.

If a string is specified as first argument, the buffer is made one item
larger than the length of the string so that the last element in the array is a
NUL termination character. An integer can be passed as second argument which
allows to specify the size of the array if the length of the string should not
be used.

Try to find a library and return a pathname. name is the library name
without any prefix like lib, suffix like .so, .dylib or version
number (this is the form used for the posix linker option -l). If
no library can be found, returns None.

Windows only: Returns the last error code set by Windows in the calling thread.
This function calls the Windows GetLastError() function directly,
it does not return the ctypes-private copy of the error code.

This function resizes the internal memory buffer of obj, which must be an
instance of a ctypes type. It is not possible to make the buffer smaller
than the native size of the objects type, as given by sizeof(type(obj)),
but it is possible to enlarge the buffer.

Windows only: this function is probably the worst-named thing in ctypes. It
creates an instance of OSError. If code is not specified,
GetLastError is called to determine the error code. If descr is not
specified, FormatError() is called to get a textual description of the
error.

Changed in version 3.3: An instance of WindowsError used to be created.

This function returns the wide character string starting at memory address
address as a string. If size is specified, it is used as the number of
characters of the string, otherwise the string is assumed to be
zero-terminated.

This non-public class is the common base class of all ctypes data types.
Among other things, all ctypes type instances contain a memory block that
hold C compatible data; the address of the memory block is returned by the
addressof() helper function. Another instance variable is exposed as
_objects; this contains other Python objects that need to be kept
alive in case the memory block contains pointers.

Common methods of ctypes data types, these are all class methods (to be
exact, they are methods of the metaclass):

This method returns a ctypes instance that shares the buffer of the
source object. The source object must support the writeable buffer
interface. The optional offset parameter specifies an offset into the
source buffer in bytes; the default is zero. If the source buffer is not
large enough a ValueError is raised.

This method creates a ctypes instance, copying the buffer from the
source object buffer which must be readable. The optional offset
parameter specifies an offset into the source buffer in bytes; the default
is zero. If the source buffer is not large enough a ValueError is
raised.

This method adapts obj to a ctypes type. It is called with the actual
object used in a foreign function call when the type is present in the
foreign function’s argtypes tuple; it must return an object that
can be used as a function call parameter.

All ctypes data types have a default implementation of this classmethod
that normally returns obj if that is an instance of the type. Some
types accept other objects as well.

Sometimes ctypes data instances do not own the memory block they contain,
instead they share part of the memory block of a base object. The
_b_base_ read-only member is the root ctypes object that owns the
memory block.

This member is either None or a dictionary containing Python objects
that need to be kept alive so that the memory block contents is kept
valid. This object is only exposed for debugging; never modify the
contents of this dictionary.

This non-public class is the base class of all fundamental ctypes data
types. It is mentioned here because it contains the common attributes of the
fundamental ctypes data types. _SimpleCData is a subclass of
_CData, so it inherits their methods and attributes. ctypes data
types that are not and do not contain pointers can now be pickled.

This attribute contains the actual value of the instance. For integer and
pointer types, it is an integer, for character types, it is a single
character bytes object or string, for character pointer types it is a
Python bytes object or string.

When the value attribute is retrieved from a ctypes instance, usually
a new object is returned each time. ctypes does not implement
original object return, always a new object is constructed. The same is
true for all other ctypes object instances.

Fundamental data types, when returned as foreign function call results, or, for
example, by retrieving structure field members or array items, are transparently
converted to native Python types. In other words, if a foreign function has a
restype of c_char_p, you will always receive a Python bytes
object, not a c_char_p instance.

Subclasses of fundamental data types do not inherit this behavior. So, if a
foreign functions restype is a subclass of c_void_p, you will
receive an instance of this subclass from the function call. Of course, you can
get the value of the pointer by accessing the value attribute.

Represents the C char* datatype when it points to a zero-terminated
string. For a general character pointer that may also point to binary data,
POINTER(c_char) must be used. The constructor accepts an integer
address, or a bytes object.

Represents the C wchar_t datatype, and interprets the value as a
single character unicode string. The constructor accepts an optional string
initializer, the length of the string must be exactly one character.

Concrete structure and union types must be created by subclassing one of these
types, and at least define a _fields_ class variable. ctypes will
create descriptors which allow reading and writing the fields by direct
attribute accesses. These are the

A sequence defining the structure fields. The items must be 2-tuples or
3-tuples. The first item is the name of the field, the second item
specifies the type of the field; it can be any ctypes data type.

For integer type fields like c_int, a third optional item can be
given. It must be a small positive integer defining the bit width of the
field.

Field names must be unique within one structure or union. This is not
checked, only one field can be accessed when names are repeated.

It is possible to define the _fields_ class variable after the
class statement that defines the Structure subclass, this allows to create
data types that directly or indirectly reference themselves:

classList(Structure):passList._fields_=[("pnext",POINTER(List)),...]

The _fields_ class variable must, however, be defined before the
type is first used (an instance is created, sizeof() is called on it,
and so on). Later assignments to the _fields_ class variable will
raise an AttributeError.

It is possible to defined sub-subclasses of structure types, they inherit
the fields of the base class plus the _fields_ defined in the
sub-subclass, if any.

An optional sequence that lists the names of unnamed (anonymous) fields.
_anonymous_ must be already defined when _fields_ is
assigned, otherwise it will have no effect.

The fields listed in this variable must be structure or union type fields.
ctypes will create descriptors in the structure type that allows to
access the nested fields directly, without the need to create the
structure or union field.

The TYPEDESC structure describes a COM data type, the vt field
specifies which one of the union fields is valid. Since the u field
is defined as anonymous field, it is now possible to access the members
directly off the TYPEDESC instance. td.lptdesc and td.u.lptdesc
are equivalent, but the former is faster since it does not need to create
a temporary union instance:

It is possible to defined sub-subclasses of structures, they inherit the
fields of the base class. If the subclass definition has a separate
_fields_ variable, the fields specified in this are appended to the
fields of the base class.

Structure and union constructors accept both positional and keyword
arguments. Positional arguments are used to initialize member fields in the
same order as they are appear in _fields_. Keyword arguments in the
constructor are interpreted as attribute assignments, so they will initialize
_fields_ with the same name, or create new attributes for names not
present in _fields_.